I built a TM500 mainframe tester, and updated the design. Someone might find this useful?


 

Fellow TM-500 Test Module Constructors,

I've just completed assembling my TM500 test module and after plugging it into a TM501 frame for testing, found that although the 5V indicator illuminated, the -15V and +15V indicators did not.

Probing around a bit I found a puzzling situation. The +15V base drive is routed to the TM501 frame PNP transistor: the one the left side as viewed through the opening from the front. This is labeled Q-12 on the circuit board, near where the leads connect. The transistor on the right side is an NPN and is labeled as Q10 on the circuit board where the leads connect.

The TM501A manual found on TEKWIKI clearly shows Q10 (PNP) on the left and Q12 (NPN) on the right.

I am now wondering if this is traceable to a Tektronix documentation error or whatever.

Has others assembling these modules run into this enigma? I have a TM-502 and a TM504, but thought I would ask first before making any changes or checking into the larger TM-units.

Bruce, KG6OJI


Jared Cabot
 

I just checked my TM501 (Not TM501A) modules, all four of them in my collection have Q12 on the left, Q10 on the right.

I also just plugged my tester module in to each and the +15 and -15 LED's lit ok when the rotary switch is set to Position 3 (+15V should be out when in Position 1, -15V should be out when in Position 4)


 

I resolved the problem. My confusion was cased by the Tek 070-1304-00 August 1974 manual for the TM501 found on TEKWIKI. This earlier manual shows Q10 on the left and Q12 on the right. The later 070-1304-01 June 1985 revision found on BAMA agrees with the configuration of my unit. Either the earlier TM501 units were configured differently or the original Tek documentation was in error.

It turned out the 2000 uFd filter capacitor for the +33.5V supply was completely open, causing the TM501 to output a half-wave rectified waveform. This destroyed the 741 op amps in both 15V supplies. I installed sockets for the replacements.

This exercise impressed me with the importance of testing these aged frames before plugging-in valuable modules. Fortunately, I did not try and use this one until now. My thanks again to Jared Cabot for this useful tool.

Bruce, KG6OJI


Larry McDavid
 

Why did the open filter capacitor destroy the 741s? Surely open filter capacitors is a problem the tester should identify and not itself fail from.

Larry

On 3/16/2021 12:01 PM, ebrucehunter via groups.io wrote:
I resolved the problem. My confusion was cased by the Tek 070-1304-00 August 1974 manual for the TM501 found on TEKWIKI. This earlier manual shows Q10 on the left and Q12 on the right. The later 070-1304-01 June 1985 revision found on BAMA agrees with the configuration of my unit. Either the earlier TM501 units were configured differently or the original Tek documentation was in error.
It turned out the 2000 uFd filter capacitor for the +33.5V supply was completely open, causing the TM501 to output a half-wave rectified waveform. This destroyed the 741 op amps in both 15V supplies. I installed sockets for the replacements.
This exercise impressed me with the importance of testing these aged frames before plugging-in valuable modules. Fortunately, I did not try and use this one until now. My thanks again to Jared Cabot for this useful tool.
Bruce, KG6OJI
--
Best wishes,

Larry McDavid W6FUB
Anaheim, California (SE of Los Angeles, near Disneyland)


 

I can only speculate why the 741s didn't like the unfiltered rectifier output as +Vs, but suspect the capacitor at the (+) input may have caused excessive input current as the device was violently cycled. As both 741s failed, there seems to be a weakness. A 741 Spice model might provide insight.

Now to investigate the DM502 and DM504 I haven't used in years and find a source for replacement capacitors.

Bruce, KG6OJI
-----------------
Why did the open filter capacitor destroy the 741s? Surely open filter capacitors is a problem the tester should identify and not itself fail from.

Best wishes,

Larry McDavid W6FUB
Anaheim, California (SE of Los Angeles, near Disneyland)Larry


Renée
 

most likely the peaks on V+ ( or inputs)  exceeded max for the 741...had it happen a lot on equipment in implanters....we also had walking wounded which would sometimes work and well sometimes do funny things...PIA to find at times so down time and my time was expensive and the part cheap....shotgun approach worked...also adding mov to the supply rails helped.
Renée

On 3/17/21 7:04 AM, ebrucehunter via groups.io wrote:
I can only speculate why the 741s didn't like the unfiltered rectifier output as +Vs, but suspect the capacitor at the (+) input may have caused excessive input current as the device was violently cycled. As both 741s failed, there seems to be a weakness. A 741 Spice model might provide insight.

Now to investigate the DM502 and DM504 I haven't used in years and find a source for replacement capacitors.

Bruce, KG6OJI
-----------------
Why did the open filter capacitor destroy the 741s? Surely open filter capacitors is a problem the tester should identify and not itself fail from.

Best wishes,

Larry McDavid W6FUB
Anaheim, California (SE of Los Angeles, near Disneyland)Larry




Jared Cabot
 

Well, I'm glad it wasn't my design that was in error. :D

I'm interested in why the opamps popped too, I wonder if there is a bodge that can be added to existing units to protect the opamps from this issue, and maybe be added into the board layout for future board revisions.

I might add a suggestion to socket the opamps in the manual too. At least they are cheap parts to replace...


Jared.


Dave Seiter
 

Just use good sockets!  I usually socket the ICs in all my projects because I hate desoldering them should the need arise, and I try to use machined sockets where possible.
-Dave

On Wednesday, March 17, 2021, 08:43:15 AM PDT, Jared Cabot via groups.io <jaredcabot=protonmail.com@groups.io> wrote:

Well, I'm glad it wasn't my design that was in error. :D

I'm interested in why the opamps popped too, I wonder if there is a bodge that can be added to existing units to protect the opamps from this issue, and maybe be added into the board layout for future board revisions.

I might add a suggestion to socket the opamps in the manual too. At least they are cheap parts to replace...


Jared.